The present invention is directed to a cement admixture and a method of using the same for the purpose of reclaiming unset cement compositions for future use. The present invention is, in particular, directed to reclaiming unset mortar compositions (a mixture of a Portland cement, small aggregate and water) or concrete compositions (a mixture of a Portland cement, small aggregate such as sand, large aggregate such as gravel and stone, and water) by introducing a specific polymethoxy diphosphonate compound to the concrete composition.
It is general practice that a concrete truck is loaded with the components required to form a concrete along with a small amount of water with the remaining water component (total water to cement ratio is normally in the range of from about 0.3 to 0.6) added at the job site. In general, excess concrete is delivered to the job site to assure that the structure is formed by a single casting. After completion of its delivery, the concrete truck must be taken to a disposal site where the excess concrete is removed from the truck and dumped. Similarly, mortar mixes formed at a job site may not be completely used during the days activities and the excess at the end of the day is normally dumped. These practices are both economically and environmentally undesirable.
Recently, U.S. Pat. Nos. 4,676,832 and 4,964,917 disclosed a method of reclaiming unset concrete by the addition of a retarding agent to the concrete. Although retarding agents are well known in the cement industry, they normally provide short periods of retardation of the set and are not subject to reversibility. However, the above patents disclose that certain specific agents are capable of giving extended set retardation and the treated concrete can be reactivated and used as part of a fresh composition. The agents disclosed are polyphosphonic acids which contain amino and/or hydroxyl groups, such as N-nitrilo tris(methylene phosphonic acid); 1,2-ethanediyl bis[nitrilo di(methylene phosphonic acid)]; 1,6-hexanediyl bis[nitrilo di(methylene phosphonic acid)] and the like. These compounds are conventionally formed from an appropriate amine or ammonia. Such reagents require special handling due to their known toxicity and corrosive nature. The compound suggested by the references are thus difficult to form and, therefore, do not provide a cost effective reclaimation means, as is required in the cost conscious cement industry.
It is highly desired to have an agent which is capable of retarding or stopping the hydration of wet cement based compositions while permitting reactivation to provide useable concrete for latter application. However, the means of achieving this desired result must be done with inexpensive, readily attainable agents, otherwise there are no economic advantages.